The present invention generally relates to continuous ink jet printing and ink compositions suitable therefor, and particularly to continuous ink jet printing and ink compositions that allow continuous printing with reduced or no make-up of the ink composition.
Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream, e.g., electronically, so that the droplets are caused to form the desired printed image on that surface.
Reviews of various aspects of ink jet printing can be found in Kuhn et al, Scientific American, Apr. 1979, 162-178; and Keeling, Phys. Technol., 12(5), 196-203 (1981). Various ink jet apparatuses are described in U.S. Pat. Nos. 3,060,429; 3,298,030; 3,373,437; 3,416,153; and 3,673,601. An example of a continuous inkjet ink composition can be found in U.S. Pat. No. 5,594,044. In general, an ink composition for use in continuous ink jet printing should possess certain desirable properties relating to viscosity, electrical resistivity, and ink dry time on the substrate. For example, a short dry time is preferable as it allows high printing speeds.
In single nozzle type continuous ink jet printing, the ink jet emanating from the nozzle breaks into small droplets. A stimulation signal is typically applied to the nozzle to form uniformly spaced ink droplets. During printing, the printing ink droplets are charged and are deflected as they pass through a region of high voltage of opposite charge. The deflected droplets are deposited onto a substrate at a location that is determined by the charging levels. The non-printing droplets are collected and returned to the ink module or tank. Generally a vacuum present in the ink tank facilitates the return of ink. In binary array type continuous ink jet printing, the printing drops are not charged and are deposited onto a substrate. The non-printing droplets are charged and deflected to a gutter and returned to the ink module or tank. The ink is thus recycled. During recycling, the ink is exposed to the ambient atmosphere and/or the vacuum, and the volatile component of the ink evaporates. The evaporation of the solvent leads to an increase in ink viscosity. The increase in ink viscosity leads to a decrease in ink flow rate. This ultimately leads to deterioration in print quality or the ability to operate the printer without interruption.
The loss of solvents in continuous ink jet printing has been recognized in the industry; see, e.g., U.S. Pat. No. 3,846,141. To maintain sufficient fluidity of the ink a make-up fluid is added to the return tank to replace lost fluids; see, e.g., U.S. Pat. No. 5,711,791.
A disadvantage of this approach includes the cost associated with the incorporation of complex mechanical and electronic ink controls into the printing system. In addition, the cost of the make-up fluid can be significantly high, as make-up consumption rates of, e.g., as high as 200 g/24 hours and higher have been experienced in continuous ink jet printing. Further, the evaporation of certain volatile solvents into the atmosphere may require ventilation considerations that at times are not desirable to the customer.
U.S. Pat. No. 5,980,624 attempts to solve the problem of solvent loss from inks employed mainly in stamp pads and ball point pens. Inks employed in these applications are distinct from continuous ink jet inks. Continuous ink jet inks have properties, e.g., viscosity, and requirements, that are distinct or more stringent than those of stamp pad or ball point inks. In as much as the ""624 patent mentions ink jet ink, it does not mention or address the concerns relating to continuous ink jet inks.
Thus, there exists a need for a continuous ink jet ink composition that has a reduced evaporation rate. There further exists a need for an ink composition that allows operation of the continuous ink jet printer with reduced or no make-up and that provides short dry time of the ink on the substrate. There further exists a need for a continuous ink jet printing process wherein ink make-up is reduced or eliminated.
The advantages of the present invention will be apparent from the detailed description of the preferred embodiments of the invention set forth below.
Many of the foregoing needs have been fulfilled by the present invention which provides an ink jet ink composition suitable for use in continuous ink jet printing. In an embodiment, the ink composition comprises a liquid vehicle, one or more binder resins, and one or more colorants. Preferably the ink composition contains a surfactant. The present invention further provides a method for eliminating or reducing make-up in continuous ink jet printing. An important aspect of the ink composition of the present invention is that make-up consumption is reduced or eliminated without significantly increasing ink dry time on a substrate. The present invention further provides a process of continuous ink jet printing which is free or substantially free of make-up.
While the invention has been described and disclosed below in connection with certain preferred embodiments and procedures, it is not intended to limit the invention to those specific embodiments. Rather it is intended to cover all such alternative embodiments and modifications as fall within the spirit and scope of the invention.